My ex-wife was a US air traffic controller, and the training they go through is stunning. They're tested continuously for years, with something like an 80% wash-out rate.
During testing, they get 1-2 tests per week, working a shift just like a controller would, with fake pilots on the radio, and real computer systems. If they let airplanes get even remotely close to each other, they fail. If it happens twice, they're out of the FAA, immediately, permanently. One of the instructors told me, "It's my job to make sure your wife has the worst week of her career, every week, during training - so that when she's actually on the job, every week will seem easy no matter what she's facing."
Even just in training, you could see the stress take its toll on the candidates over the years.